The present application claims the priority of a United Kingdom patent filed 09/13/2000 under application No. 0022503.7.
The present invention relates to a microwave holographic measuring method and apparatus, and relates particularly, but not exclusively, to such method and apparatus for testing a microwave antenna or forming an image of an object.
Methods of testing microwave antennas by microwave holography are known and an arrangement for carrying out such a method is shown in FIG. 1. A signal from a network analyser 1 causes an antenna 2 under test to emit microwave radiation, which is sampled at suitable intervals over an appropriate aperture by means of a sampling antenna 3 which can be moved along the axes X and Y shown in the figure. The output signal from the sampling antenna 3 is returned to the network analyser. At each sampling position, the signal radiated from the antenna 2 under test is fed to the network analyser 1, which determines the relative amplitude and phase of the return signal by comparison with the output signal of the network analyser. These values of amplitude and phase at each scanning position are recorded, and can be Fourier transformed to produce patterns of antenna radiation.
However, this known method suffers from the drawback that the cost of the network analyser is very high, and the network analysers are restricted in frequency, as a result of which the range of applications of the method is fairly limited.
Preferred embodiments of the present invention seek to overcome the above disadvantages of the prior art.
According to an aspect of the present invention, there is provided a microwave holographic measuring method comprising:
providing a first electrical signal of at least one microwave frequency;
directing a first part of said first signal to a first antenna;
applying predetermined changes of phase and amplitude to a second part of said first signal to produce a second electrical signal, wherein said second part is coherent with said first part;
detecting microwave radiation at a plurality of locations by means of a second antenna to generate a respective third electrical signal at each said location; and
combining said second and third signals to produce a fourth electrical signal.
By applying predetermined changes of phase and amplitude to part of the first electrical signal, this provides the advantage that the second electrical signal can reproduce the behaviour of reference microwave radiation which would interfere with the radiation emitted by the antenna under test, i.e. by electrically imitating the behaviour of interfering microwave radiation. As a result, a hologram of the radiation pattern observed at the sampling antenna can be produced, which avoids the necessity for a network analyser. This reduces the cost of equipment for carrying out the method, and also enables the apparatus to operate over a wider range of frequencies or even simultaneous operation at multiple frequencies, which in turn broadens the range of applications of the method. The invention also has the advantage that by electrically imitating the behaviour of interfering microwave radiation, this under some circumstances permits the electrical synthesis of reference microwave radiation which cannot be produced in the form of microwaves.
The predetermined changes of phase and amplitude may be chosen to reproduce a predetermined microwave signal at each said location.
The method may be a method of measuring radiation characteristics of the first antenna.
The method may further comprise the step of comparing values of said fourth electrical signal with corresponding values of the fourth electrical signal for an antenna of known characteristics.
This provides the advantage of enabling the properties of antennae to be tested by comparing unprocessed image holographic data, as opposed to processed data, thus offering savings in efficiency.
In a preferred embodiment, the method is a method of forming a microwave image of an object, and further comprises the step of illuminating the object with said first part.
In a preferred embodiment, the predetermined changes of phase and amplitude are chosen to reproduce a predetermined microwave signal at each said location.
The method preferably further comprises the step of combining said fourth signal with a signal representing predetermined microwave radiation to generate holographic image data for each said location.
The method may further comprise the step of applying predetermined changes of phase and amplitude to said fourth electrical signal to produce a fifth electrical signal, wherein said fifth electrical signal represents a microwave image of the object.
The method may further comprise the step of processing said fourth electrical signal to generate data representing an image of the object at locations other than said plurality of locations.
The method may be a method of detecting buried objects.
According to another aspect of the present invention, there is provided a microwave holographic measuring apparatus comprising:
signal generator means for generating a first electrical signal of at least one microwave frequency;
coupler means for directing a first part of said first signal to an antenna and providing a second part of said first signal, wherein said second part is coherent with said first part;
phase/amplitude adjusting means for applying predetermined changes of phase and amplitude to said second part to produce a second electrical signal;
at least one first antenna for detecting microwave radiation at a plurality of locations to generate a respective third electrical signal at each said location; and
combining means for combining said second and third signals to produce a fourth electrical signal.
The apparatus may further comprise at least one second antenna for receiving said first part of said first signal and illuminating an object.
The apparatus may further comprise detector means for detecting said fourth signals.
The apparatus preferably further comprises position adjusting means for adjusting the position of the or each said second antenna.
The apparatus may further comprise controller means for controlling said phase/amplitude adjusting means.